Focus control circuits for adjusting the focal point of an electron beam on a screen of a CRT display are well known in the art. In a CRT display unit, there is an electrode (grid) for forming an electron lens using the electrostatic effect among a plurality of which form an electron gun. The converging capability of the electron lens is changed, that is, the magnification is changed, by changing the level of a voltage applied to the electrode (grid).
With reference to FIG. 1, construction of an electron lens is briefly described. An electron beam EB discharged from cathode K is concentrated by a control grid G1 and an accelerating grid G2, and a crossover point P is formed. An electron lens 1 concentrates an image of the crossover point P on a fluorescent screen 2, and is controlled by a focus bias voltage applied to focus grids G3 and G4.
An example of a conventional focus bias circuit is illustrated in FIG. 2. In the case of UPF (Unipotential Focus), a +B1 voltage source of approximately 1 kV and a -B2 voltage source of approximately -1500 V are used. For example, a character image of 80 NIT luminance is displayed and a variable resistor VR1 for controlling the focus is adjusted by using the human eyes or a microscope, so that the character image is focused on the fluorescent screen.
The characteristics required for an electron lens or an electron lens system are almost the same as those required for a general optical lens, such as that used in a camera. An attempt is made to decrease aberration by arranging lenses in multiple stages or using a lens of large aperture. Electron guns of different type have been suggested.
The arrangements mentioned above contributed mainly to an improvement in focusing of a motionless spot on the tube axis of a CRT. However, when scanning a problem associated with changes in the focal length appears.
With reference to FIG. 1 again, when a motionless spot on the tube axis of a CRT is best focused, magnification of the electron lens is determined by the following two factors: 1. the distance "a" between the crossover point P and the electron lens 1; and 2. the distance "b" between the electron lens 1 and the fluorescent screen 2. However, as shown in FIG. 3, the distance b differs between that for a spot on the tube axis and that at an end or edge portion of the fluorescent screen 2. The flatter the screen of the CRT, the longer is the distance b at the edges. In FIG. 3, the distance b is greater at the edge of the fluorescent screen 2 than at the spot on the tube axis, by the distance indicated by the symbol "c".
Thus, it is apparent that the focus bias voltage to be applied to each grid for obtaining the optimum spot on the tube axis must be somewhat corrected for positions other than on the tube axis. Specifically, when magnification "MO" on the tube axis is b/a, magnification "MI" at the edge in FIG. 3 is (b+c)/a.
Consequently, a conventional technique called "dynamic focus control" has been developed by which the focus bias voltage is automatically corrected for the beam scanning position on a fluorescent screen. Many treatises have been written and inventions have been disclosed in this area.
When the dynamic focus control technique is applied, a focus deviation is generated. The amount of the focus deviation differs somewhat with CRT types. For example, when a CRT of 15 inch flat square type is used, "a" is equal to approximately 25 mm and "b" is equal to approximately 310 mm.
The edge of the fluorescent screen is perpendicular to the tube axis and is 130 mm away from the tube axis. The radius of curvature is 1200 mm, and the center of curvature is at "O". In this case, "c" is equal to approximately 20 mm.
Consequently, in the case of this CRT, EQU M0=310/25=12.4 (1) EQU M1=(310+20)/25=13.2
The variation in magnification for correcting c by the dynamic focus control is: EQU ((M1/M0)-1)100.apprxeq.6.5% (2)
In this conventional technique, in addition to focus adjustment at predetermined luminance, dynamic focus control must be performed.
Consequently, as described below, when luminance is changed after the best focus has been obtained by adjusting a focus bias voltage at a predetermined luminance, the focus deviates. The conventional technique cannot correct this focus deviation.